1. Field of the Invention
The invention pertains to highly pigmented aqueous coating compositions with improved resistance to blocking, and to a method of improving blocking resistance through the addition of a functional organopolysiloxane emulsion to a highly pigmented aqueous coating composition.
2. Background Art
Pigmented aqueous coating systems (“paints”) have been in use for many years now. These systems contain a film-forming organic binder in finely dispersed form, the organic binder generally being thermoplastic or thermosetting in nature. In some cases, combinations of thermoplastic and thermosetting binders are used, or essentially thermoplastic binders containing a limited amount of crosslinkable functional groups are used. After the coating has been applied, the organic binders coalesce to form a film as the aqueous phase evaporates. In the case where crosslinkable functionalities are present, these may begin to crosslink at this point, or crosslinking may take place very slowly, for example under the influence of atmospheric oxygen or of light.
The chemistries of aqueous coating systems vary widely with the end use application. For example, in interior and exterior house paints, the so-called “latex paints,” the organic binder may be a relatively simple addition polymer, such as polyvinyl acetate, copolymers of ethylene and vinyl acetate, and polymers derived from other monomers, such as acrylates, methacrylates, vinyl chloride, styrene, etc., often with functional groups such as carboxylic acid groups, anhydride groups, epoxy groups, etc. In the automotive sector, on the other hand, the chemistry is much different, and may include polyurethane resins, aminoplast resins, melamine formaldehyde resins, and complex mixtures of these.
When the aqueous coating system is desired to be opaque, and in particular both opaque and colored, pigments are added. In some paints, particularly latex house paints, some opacity may be provided by the thermoplastic binder particles themselves. However, in most cases, pigments are required to be added, at times in high quantity.
The pigments actually perform several functions in aqueous coating systems. For example, incorporation of pigments may increase the “body” of the coating (“paint”), and may increase its resistance to sagging and running Very fine particles are not as effective as larger particles in opacifying power or pigmentary power, but may serve to alter the rheology of the coating, for example in terms of thixotropy or dilatency, or in Newtonian systems, by simply increasing viscosity.
For highly opaque coatings, especially highly opaque and highly colored systems, a very high pigment loading is necessary. However, with high pigment loadings, a complication arises in that the dry coatings tend to block. The reason why highly pigmented aqueous coatings develop blocking behavior is not completely understood and is even somewhat counterintuitive, but this behavior is quite undesirable, as dry but freshly painted surfaces will be much more likely to attract dirt and dust, and dry but freshly painted surfaces may stick to each other or to surfaces they abut, a phenomenon called “blocking” This is especially a problem in industrial manufacturing, where painted articles of metal, plastic, wood, etc. must then be handled without the painted surfaces being contacted by other surfaces.
It would be desirable to reduce the blocking behavior of highly pigmented aqueous coating systems without interfering with the cure or drying of these systems and without affecting the uncured system stability.